Permanent magnets are one of the most basic types of magnets that exist today. Essentially, permanent magnets are metal alloys that are capable of retaining at least some degree of magnetization. The metal alloys are initially magnetized by exposure to a magnetic field of an external source, and in turn retain the magnetization for relatively long periods of time. Common metal alloys used as permanent magnets include Neodymium-Iron-Boron (NdFeB), Samarium-Cobalt (SmCo), Aluminum-Nickel-Cobalt (Alnico), or the like. As each metal alloy exhibits a different characteristic, one metal alloy may be preferred over another based on the application. For instance, the alloy NdFeB provides the highest magnetic field strength with respect to other alloys. Accordingly, NdFeB is preferred for more critical applications requiring stronger magnetic fields for very dense and or compact assemblies.
Permanent magnet assemblies are used in a wide variety of applications including many industrial and scientific processes. While each application may vary, permanent magnets are increasingly being used for more delicate applications involving compact assemblies, in-vacuum processes, high levels of radiation, and any combinations thereof. For such conditions, it is important to consider the size, the resistance to radiation, and the rate of outgassing of the magnet assemblies. Accordingly, efforts to prepare permanent magnet assemblies for use in such conditions have intensified.
Permanent magnets may be assembled in a number of ways. One such method simply fastens permanent magnet assemblies together mechanically. However, mechanical attachments tend to occupy more space, which may cause problems for dense and compact applications. Gluing or using adhesives is a more compact way to prepare permanent magnet assemblies. Although adhesives take up significantly less space than mechanical fixtures, adhesives are susceptible to radiation and may easily fall apart when exposed to high levels of radiation. Furthermore, adhesives are also a major source of outgassing and gradually release significant amounts of gas into the surrounding environment. This is a major problem for in-vacuum assemblies because the released gas may condense onto and distort functionality of optical elements, thermal radiators, solar cells, or other vital equipment.
Another method typically used for such assemblies involve soldering. Soldering takes less space to implement than mechanical fixtures and is ideal for more compact assemblies. It is also more resistive to radiation and exhibits significantly smaller rates of outgassing than adhesives, which are ideal characteristics for high temperature and in-vacuum applications. However, the high temperatures involved in soldering adversely affect the magnetic properties of a permanent magnet. More specifically, exposing a magnet to temperatures exceeding its maximum operating temperature, can easily demagnetize the magnet, or at the least, significantly distort its magnetic properties.
Referring to the table of FIG. 1A, magnetic characteristics of common NdFeB alloys that may be used to form a permanent magnet are provided. For instance, the magnetic characteristics of a N40SH alloy are shown to have a maximum energy product of 41×106G·Oe, a maximum operating temperature of 150° C. and a demagnetization Curie temperature of 340° C. Accordingly, if the N40SH alloy is exposed to temperatures ranging between 150° C. and 340° C., its magnetic properties are distorted and a full recovery is not possible. If the N40SH alloy is exposed to temperatures exceeding 340° C., the alloy is demagnetized and no recovery is possible.
The graphs of FIGS. 1B and 1C further define various relationships between the magnetic properties of the N40SH alloy and temperature. In general, the demagnetization curves B1-B6 of FIG. 1B illustrate the changes in the magnetic flux density and magnetic field intensity with respect to changes in temperature. The plot of FIG. 1C summarizes the linear relationship between the demagnetization temperature and the minimum of the magnetic field flux density in the alloy. In the graph of FIG. 1B, demagnetization characteristics are provided for different temperatures, for example 23° C. (curve B1), 60° C. (curve B2), 100° C. (curve B3), 120° C. (curve B4), 150° C. (curve B5) and 180° C. (curve B6). Furthermore, some of the demagnetization curves B3-B6 have knees K3-K6, respectively. The knees of curves B1 and B2 have been cutoff and are not shown in FIG. 1B. The projection of the knees K3-K6 on the right axis represents the minimum magnetic field density for a given alloy at the respective temperatures. For instance, the demagnetization curve B4 at 120° C. has a knee K4 at a magnetic field flux density of approximately 0.35T (3.5 kGs). This means that, at 120° C., demagnetization will occur if the magnetic field density inside the permanent magnet is reduced to 0.35T (3.5 kGs) or less. This further implies that, if the magnetic field flux density inside the permanent magnet is reduced to 0.35T (3.5kGs), the magnet will be demagnetized at a temperature of 120° C.
Typical solder, such as a 63/37 Tin-Lead alloy, melts at temperatures of approximately 180° C. To ensure the solder has melted completely, the heat provided to melt the solder may exceed well beyond the rated melting point. For instance, soldering irons typically provide heat of approximately 250° C. for applying a type of solder having a 190° C. melting point. Alternatively, ovens may be used to bake the solder at temperatures closer to, but still exceeding, the solder melting point. In any case, soldering temperatures still exceed maximum demagnetization limits of typical NdFeB magnets. More specifically, soldering a N40SH magnet may heat the magnet to temperatures well above its maximum operating temperature of 120° C., and consequently demagnetize, or at the least, distort the magnetic properties of the magnet even long after the heat is removed.
In light of the foregoing, there is a need for a method that modifies a permanent magnet so as to increase its demagnetization temperature, thereby allowing the permanent magnet to undergo high-temperature processes, such as soldering, without affecting its magnetic properties.